1. Wider Audience - Kindle Direct Publishing can place your book on thousands of lists and search engines around the globe. They can format your book for Kindle, but I recommend uploading to Nook and Smashwords as well. Kindle is by far the easiest interface.

2. Createspace Print Edition - For absolutely no cost, you can publish your book as a printed edition (if you use their cover designer and format things properly) or you can pay a measely $39 to have the "premium" version which simply gives you a little more control as to how it is distributed.

3. Multiple Currencies - Your book can be sold in several countries and will be priced using that country's currency which makes it easier for people in those countries to buy the book.

4. Ease of Use - Kindle Direct Publishing is probably the easiest service to use because all you do is upload a .doc file of the interior and play around with the cover editor if you use Createspace (which I recommend). You can also use the handy Scrivener program to format directly to a Kindle .mobi file and save Amazon the trouble (which isn't much for them).

If you would like more advice about selling your book on Amazon and marketing it smartly, check out this link to a great article on the subject which will lead you to one of the best books about it.